1. Field of the Invention
The present invention relates, generally, to sample-holding mechanisms for analytical use and, more particularly, to sample-holding mechanisms and methods for use with optical analyzing instruments.
2. Description of the Related Art
Since the mid-sixties, optical analysis employing near-infrared (NIR) light energy has increasingly become the preferred method of determining the composition and constituent concentration of a sample, particularly grain. This technique is replacing the traditional, slightly more accurate, chromatographic or indirect separation chemical techniques which are generally substantially more time consuming. As compared to these traditional techniques, the current optical analyzing assemblies offer a diminishing precision gap, substantially shorter analyzing time intervals and a reduction of skilled personnel necessary for operation. Amongst other things, these factors account for the optical analyzing assemblies' commercial success. One particularly successful optical analyzer, for example, is the INFRAMATIC 8600.TM. by Perten Instruments North America, Inc. which is a rapid, non-destructive, spectrophotometer designed for simultaneous multi-component analysis for most types of flour samples.
Briefly, it has been discovered that the chromophores composing a sample actively respond to at least one particular light energy wavelength, NIR, in general. More specifically, the absorptivity of the sample, as determined by quantitative diffuse reflectance or transmissivity of the preselected wavelength light incident on the sample, is a function of its composition and constituent concentration. Through correlation spectroscopy algorithms together with the appropriate spectroscopic measurements, the constituent concentrations, such as protein, moisture, starch and oil in grain, may be determined.
To analyze a grain specimen using light energy correlation spectroscopy, the sample is placed in a sample holder or cuvette and positioned in the optical pathway of light energy of a preselected wavelength. Typically, the range of preselected wavelength light energy corresponds to the NIR ranges which is emitted from a light source, such as wide band wavelength quartz tungsten-halogen light source. Subsequently, the absorptivity is measured by one of two types of NIR quantitative analysis instruments commercially available. One type measures the sample diffuse reflectance while the other measures the sample transmissivity. In either type, the reflectivity or transmissivity of the preselected wavelength light incident on the sample, as set forth above, is a function of its constituent concentration.
Optical analysis is particularly effective for quantitative compositional measurements on homogenous substances. For these substances, only a small particulate is necessary to compositionally represent the entire source. Such a small sample, because of its homogeneity, will not vary in composition from sample to sample. This is particularly useful when the concentration needs to be determined quickly, or when only a small quantity exists.
In contrast, when a sample is of a heterogenous composition, such as whole or ground grain, often a small sample is not truly representative of the entire composition. Therefore, a series of sub-measurements on a single sample or measurement of multiple sub-samples are necessary to cumulatively determine the composition and constituent concentrations. Depending on the accuracy required, the number of sub-measurements or of sub-samples is a function of the particle size of the sample and the variation in composition among the particles within the bulk.
Generally, the introduction of the sample or sub-sample into the optical pathway of the light source for irradiation is performed through manual positioning and repositioning. This involves the manual placement of the cuvette between the light source and a detection device to measure the diffuse reflectance or transmissivity. When a single sample is used for heterogenous compositions, the operator must manually reposition the sample, if multiple measurement are to be performed, to assure representation of the bulk. Regardless of the method (i.e., single sample or multiple sub-sample), such operator dependance, however, is often subject to a variety of procedural errors such as varying total procedure time which introduces drift errors; alignment errors which compound with multiple manual positioning operations; and potential operator indifference to complicated procedures. Moreover, reproducibility or duplication of the environmental conditions between each sub-sample, which adversely affects precision, is difficult to maintain. Such operational and procedural errors are fairly common.
Another positioning technique is to mechanically position the sub-samples or single sample in the optical pathway for irradiation. Some of these devices mechanically move the sample along the optical pathway to irradiate different areas of the sample specimen.
While these devices adequately and mechanically position or reposition the sample in a variety of locations with respect to the optical pathway, several inherent problems merit discussion. Generally, these apparatus are fairly complex units. Often these apparatus provide independent cuvettes or sample holding containers which are to be placed in complex tracking mechanism formed to move the cuvette past the analyzing window. To assure that the cuvette does not scratch or damage the analyzing window, the tracking mechanism must provide sufficient support to the cuvette so that contact is not made.
Typical of such an effort is the mechanical sample holder mechanism disclosed in U.S. Pat. No. 4,692,620 to Rosenthal. In this device, a cuvette filled with a specimen includes transparent viewing sides holding the specimen therein. The cuvette is coupled to a mechanical drive cam and positioned inside a tracking mechanism which facilitates movement of the cuvette across an analyzing window for irradiation by the optical analyzer. The cam further provides support to the cuvette so that there is no severe contact between the transparent surfaces of the cuvette and the analyzing window. In addition, the tracking mechanism fully encompasses the cuvette which discourages replacement or removal of the cuvette.
Moreover, preparation of individual samples or sub-samples for these apparatus, as well as for manually positioned sub-samples, is a tedious and labor intensive task. The operator must uniformly prepare and position the specimen into the cuvette to assure optimum optical quality and duplication between the sub-samples. Proper measurement procedure requires the specimen to be optically dense. Thus, not only must the specimen be firmly positioned in the cuvette, but, amongst other factors, this depends on the composition, optical characteristics of the composition and particle size.
Typically, a small quantity of the specimen is placed in a cuvette having two spaced apart, substantially parallel walls, at least one of which represents a transparent medium. Accordingly, if the sample holder is to be designed to hold a variety of specimens, to assure proper optical density, it is advantageous to provide a means for varying the depth of the specimen. This may be done by providing adjacent substantially parallel walls, as discussed above, which can be variable spaced. Rosenthal, for example, provides a movable cuvette capable of varying the specimen depth by providing movable transparent panels. Generally, these cuvettes are complex and require time consuming and labor intensive sample preparation.
More importantly, however, when multiple sub-samples are required or when multiple measurements are to sequentially performed at various location on a single sample, the optical characteristics and properties of each transparent medium location may need to be accounted for. This is especially true when precision is a primary consideration. In these instances, each transparent media location, even within the same sample, exhibits independent absorbent and reflective properties which increase the overall uncertainty in the measurements. Often three or four different sub-samples, or even three or four different locations on the same sub-sample, are required to attain the required accuracy, each of which feature independent optical characteristics. For precise constituent concentration determinations, these uncertainties are reduced by individually characterizing each media location, by determining a calibration or error factor, to compensate for its individual optical characteristics. Ultimately, such individual transmission properties may slightly vary the intensity of the incoming radiant light energy impinging the specimen. This may cause complications for low intensity irradiations of a sample, especially when multiple cuvettes are to be employed. Moreover, oily or wet samples often leave a residue on the cuvette windows which affects subsequent optical measurements.
This is particularly problematic with spectroscopic analyzing assemblies employing sample changer apparatus. Such a device is disclosed in U.S. Pat. No. 4,695,727 to Brierley. This device discloses a sample changer apparatus holding a plurality of sub-sample specimens which are automatically positioned in the optical pathway of the light source. As mentioned, when precision is desirable, each transmission location must be individually characterized which is not only labor intensive, but also highly prone to error when cuvette are removed and refilled, or broken and replaced.
Accordingly, it is an object of the present invention to provide a sample holder apparatus and method for use with an optical analyzing assembly which facilitates more precise and convenient spectroscopic analysis measurements.
It is another object of the present invention to provide a sample holder apparatus for use with an optical analyzing assembly which eliminates the need for preparing specimens of heterogeneous substances.
Still another object of the present invention is to provide a sample holder apparatus for use with an optical analyzing assembly which substantially reduces operational and procedural errors.
Yet another object of the present invention to provide a sample holder apparatus for use with an optical analyzing assembly which substantially reduces the need for individually characterizing each sample container or sample container position.
It is a further object of the present invention to provide an optical analyzing apparatus and method which is durable, compact, easy to maintain, has a minimum number of components, is easy to use by unskilled personnel, and is economical to manufacture.
The apparatus and method of the present invention has other objects and features of advantage which will be more readily apparent from the following description of the Best Mode of Carrying Out the Invention and the appended claims, when taken in conjunction with the accompanying drawing.